Distribution and frequency of resistance to four herbicide modes of action in Lolium rigidum Gaud. accessions randomly collected in winter cereal fields in Spain

Lolium rigidum is the most prevalent and damaging grass weed of winter cereals in Spain. L. rigidum infestations are frequently treated with herbicides and, consequently, populations have evolved resistance. The objective of this study was to determine the extent and frequency of herbicide resistance in L. rigidum populations in Spain to the selective herbicides chlortoluron, diclofop-methyl and chlorsulfuron, commonly used for its control in-crop, and to glyphosate. The response to these herbicides was evaluated on 123 accessions surveyed randomly across cereal cropping areas of the regions of Castile and León, Catalonia and Andalusia. The fresh weight and the frequency of undamaged plants were calculated for each accession and herbicide. At the regional level, higher frequencies of accessions displaying resistance occurred in Catalonia, an intensively cropped region with a greater herbicide selection pressure. Of concern is that in this region the 60% of the accessions displayed some level of resistance to the ALS-inhibiting herbicide chlorsulfuron. The 6.9% of the accessions found in Castile and León with some resistance to glyphosate could also indicate an incipient problem of resistance to this herbicide. For the other herbicides and regions the majority of the accessions remained susceptible. The possible mechanisms of herbicide resistance development in L. rigidum accessions (target-site versus non-target-site resistance) and their variation among regions was discussed. This study can be used to generate herbicide resistance-management schemes for farmers, based upon the herbicide the site and the potential for resistance development.     

Cyperus difformis evolves resistance to propanil

Cyperus difformis L. is one of the worst weeds of rice world-wide and has evolved resistance to acetolactate synthase (ALS)-inhibiting herbicides in rice fields of California. Propanil use was intensified to control the widespread resistant biotypes. Rice growers have recently experienced poor control, suggesting resistance to this photosystem II-inhibiting herbicide may have evolved in C. difformis populations. The objectives of this study were to detect the presence of propanil resistance, to establish resistance levels, and to investigate involvement of enhanced herbicide detoxification as mechanism of resistance through the use of metabolic inhibitors. Four C. difformis populations collected in rice fields from the Sacramento Valley of California were confirmed resistant to propanil. This is the first case of such resistance outside the Poaceae and the first time C. difformis exhibits resistance to an herbicide mechanism of action other than ALS inhibition. Carbaryl and malathion applied individually in mixture with propanil had minor effects on herbicide toxicity suggesting metabolic detoxification was not a resistance mechanism. A resistant biotype produced more than 80% biomass after a propanil (6.7 kg a.i. ha⁻¹) and carbaryl (1.9 kg a.i. ha⁻¹) or propanil and malathion (1.0 kg a.i. ha⁻¹) treatment compared to <20% by a susceptible biotype, suggesting substantial resistance still persisted in spite of insecticide addition. Propanil-resistant plants were cross-resistant to bensulfuron-methyl, imazosulfuron, halosulfuron-methyl and penoxsulam, but susceptible to carfentrazone. The loss of propanil to control this important weed of rice underscores the fragility of herbicide-based weed control in monoculture rice. Integrated weed management approaches to decrease herbicide selection pressure are needed to mitigate the evolution of multiple-herbicide resistance in C. difformis of California rice.     

Genetic analysis of field resistance to tan spot in spring wheat

Tan spot, caused by Pyrenophora tritici-repentis (Died.) Drechs., is an important constraint to wheat (Triticum aestivum L.) yield in many countries. Since the inheritance of field resistance to tan spot is poorly understood, this study was conducted to determine the genetic control of resistance in the field. Resistance was measured as disease severity caused by P. tritici-repentis race 1 in four crosses involving five wheat parents: parent 1 (P₁) = catbird; parent 2 (P₂) = Milan/Shanghai-7; parent 3 (P₃) = Alondra/Coc//Ures; parent 4 (P₄) = Bcb//Dundee/Gul/3/Gul); parent 5 (P₅) = ND/VG9144//Kal/BB/3/Yaco/4/Chil. P₁, P₂ and P₃ were resistant and P₄ was moderately resistant, whereas P₅ was susceptible to tan spot. The F₂-derived F₃ families and the parents were field evaluated at El Batán, Mexico, in 1996. When all the plants within a F₃ family expressed low levels of disease severity similar to that of the resistant parent it was classified resistant (R), otherwise the progeny was classified as susceptible (S). The progeny of the three crosses of the susceptible parent with the resistant and moderately resistant parents P₂, P₃, and P₄ segregated as 3R:13S whereas the progeny in the cross with P₁ showed a segregation ratio of 1R:15S. This suggests that each resistant parent possessed two genes conditioning resistance to tan spot severity caused by P. tritici-repentis race 1. Information on the inheritance of resistance measured as disease severity on adult plants under field conditions is of practical importance for wheat breeding programs seeking improvement in tan spot resistance.     

Susceptibility variation to new and established herbicides: Examples of inter-population sensitivity of grass weeds

The objectives of this study were to describe the intra-specific variation in herbicide response of weed populations when subjected to new vs. well-established herbicides, and to assess distributions of logLD₅₀- and logGR₅₀-estimates as a potential indicator for early resistance detection. Seeds of two grass weeds (Alopecurus myosuroides, Apera spica-venti) were collected in southern Sweden, mainly in 2002. In line with the objectives of the study, the collections sites were not chosen for noted herbicide failures nor for detected herbicide resistance, but solely for the presence of the target species. For each species, seedlings were subjected to two herbicides in dose-response experiments in a greenhouse. One herbicide per species was recently introduced and the other had been on the market for control of the species for a decade, with several reports of resistance in the literature. Fresh weight of plants and a visual vigour score were used to estimate GR₅₀ and LD₅₀, respectively. Resistance to fenoxaprop-P-ethyl in A. myosuroides was indicated by the LD₅₀-estimates to be present in frequencies sufficient to affect the population-level response in 9 of 29 samples, and was correlated to response to flupyrsulfuron, while low susceptibility to isoproturon in A. spica-venti populations was not linked to the response to sulfosulfuron. In the study as a whole, the magnitude of the estimated herbicide susceptibility ranges differed irrespective of previous exposure. No consistent differences were found in the distribution of LD₅₀-estimates for new and “old” herbicides, and normality in the distribution of estimates could not be assumed for a non-exposed sample, even in the absence of an indication of cross-resistance.     

Characterisation and management of Phalaris paradoxa resistant to ACCase-inhibitors

Phalaris paradoxa is a competitive grass commonly found in durum wheat crops of central and southern Italy. Among the 85 populations screened from 1998 to 2008 for resistance to ACCase-inhibitors and graminicide sulfonylureas, 17 resulted as being resistant to at least one ACCase inhibitor while none of the populations showed resistance to sulfonylureas. ACCase resistance in hood canary-grass seems to be spreading rather slowly in Italy. Out of the 17 populations, seven were characterised through outdoor dose-response pot experiments to investigate resistance levels and cross-resistance patterns to ACCase-inhibitors and multiple resistance to other mode of action. Molecular bases of resistance to the recently introduced DEN herbicide pinoxaden were also investigated. Six populations were confirmed to be ACCase-resistant with various cross-resistance patterns. Two populations were resistant to all tested ACCase herbicides, with pinoxaden resistance indexes (RI) based on survival ranging from 22 to 50. The two populations have been molecularly characterised for resistance to pinoxaden. A single point-mutation in the ACCase gene was identified in each population, causing the amino-acid substitutions of Ile₁₇₈₁Val and Asp₂₀₇₈Gly in 0478L and 0025, respectively. The results suggest that resistance of P. paradoxa to pinoxaden is due to an altered target site and different mutations cause different resistance levels. The biological characteristics of the species, mainly self-pollinated, and the absence of multiple resistance allow herbicides with different modes of action to be used for controlling ACCase-resistant populations. Chemical tools should be carefully used within integrated weed management strategies.     

Regional scale distribution of imidazolinone herbicide-resistant alleles in red rice (Oryza sativa L.) determined through SNP markers

Red rice is the main weed in rice paddy fields. Imidazolinone herbicides in resistant rice cultivars currently provide a unique opportunity to control red rice in large-scale rice fields. However, the continuous use of this technology has resulted in imidazolinone-resistant red rice biotypes. This study aimed to identify the mechanism of herbicide resistance and the frequency and spatial distribution of the known imidazolinone herbicide-resistant alleles in red rice. The nucleotide sequence of the ALS gene indicated that the G₆₅₄E, S₆₅₃D and A₁₂₂T mutations are present in the imidazolinone herbicide-resistant rice cultivars IRGA 422 CL, SATOR CL and PUITÁ INTA CL, respectively. This information and the nucleotide sequence surrounding these mutations were used for the development of single nucleotide polymorphism (SNP) molecular markers to identify the possible mutations that confer herbicide resistance in red rice. This analysis was carried out in a total of 481 plants from 38 populations collected as individuals that escaped control with the herbicides imazethapyr and imazapic in rice paddy fields in Southern Brazil. The G₆₅₄E mutation was the most frequent, being found in 100% and 90.9% of the populations in the 2006/2007 and 20007/2008 seasons, respectively. In addition, the S₆₅₃D and A₁₂₂T mutations were also present either alone or as double or triple mutations in some plants. Target site insensitivity is the predominant mechanism of resistance in red rice resistant to imidazolinone herbicides in Southern Brazil. The high frequency of the S₆₅₃D mutation, the same mutation responsible for the resistance in the rice cultivar largely used in Southern Brazil, indicates that gene flow is occurring from the rice cultivar to red rice. Management practices related to increasing crop sanitation and decreasing of herbicide selection pressure through crop rotation should be enforced to prevent the evolution of herbicide resistance in red rice.     

Biochemical characterization of acetylcholinesterase, cytochrome P450 and cross-resistance in an omethoate-resistant strain of Aphis gossypii Glover

The degree of insecticide resistance, synergism effects, acetylcholinesterase (AChE) activity kinetics, specific activities of detoxification enzymes and cross-resistance were investigated in omethoate resistant and relatively susceptible strains of Aphis gossypii Glover. The resistant cotton aphid strain (ORR) exhibited 231.3-fold resistance to omethoate compared to the susceptible strain (OSS). Synergist piperonyl butoxide (PBO) dramatically increased the toxicity of omethoate in the resistant strain, while triphenyl phosphate (TPP) and diethyl maleate (DEM) did not exhibit synergism effects. The calculated AChE activity, V_max and K_m ratios of ORR to OSS were 0.1, 0.2 and 0.4, respectively. Based on analysis of IC₅₀ indices, enzyme inhibition experiments showed that AChE from the ORR strain was 10.6-, 3.2-, 6.2-, 10.5- and 4.4-fold more insensitive to inhibition by eserine, omethoate, paraoxon, paraoxon-methyl and malaoxon, respectively, than that from the OSS strain. The cytochrome P450-mediated O-demethylation activity (3.7-fold) and ethoxycoumarin-O-deethylase activity (2.6-fold) in the ORR strain were significantly higher than in the OSS strain. Specific activity of carboxylesterase (CarE) and glutathione S-transferase (GST) were not significantly different in both the ORR and OSS strains. Bioassay results indicated the ORR strain had developed high levels of cross-resistance to chlorpyrifos (24.2-fold), malathion (21.1-fold), acephate (10.2-fold), esfenvalerate (30.6-fold), methomyl (22.4-fold), carbofuran (33.2-fold), but had negative cross-resistance to bifenthrin (0.4-fold). Overall, these results demonstrate that reduced AChE sensitivity, combined with increased cytochrome P450 monooxygenase detoxification, plays an important role in the high levels of omethoate resistance and can cause cross-resistance to other insecticides in the ORR strain.     

Toxicity, inheritance of fenpyroximate resistance, and detoxification-enzyme levels in a laboratory-selected fenpyroximate-resistant strain of Tetranychus urticae Koch (Acari: Tetranychidae)

A strain (BEYO 2) of Tetranychus urticae was selected with fenpyroximate for 14 selections. The resulting strain (named FPY 14) became resistant to fenpyroximate. The present study examines the inheritance of fenpyroximate resistance, the toxicity of some insecticides and acaricides, detoxification enzymes [esterase, glutathione S-transferase (GST), and monooxygenase (P450)], and the synergistic ratios of certain synergists [piperonyl butoxide (PBO), S-benzyl-O,O-diisopropyl phosphorothioate (IBP), and triphenyl phosphate (TPP)] in the FPY 14 strain of T. urticae. A spray tower-Petri dish method was used in the selection and toxicity studies. The level of fenpyroximate resistance was 64.43-fold higher in the FPY 14 strain and 1.06-fold higher in the BEYO 2 strain compared to the GSS (German susceptible strain) strain. The FPY 14 strain was 7.80-, 6.90-, 6.43-, 4.78- and 2.78-fold more resistant to abamectin, chlorpyrifos, propargite, clofentezine and amitraz, respectively. Fenpyroximate resistance is inherited as an incompletely dominant trait with no sex linkage. None of the synergists showed a significant synergistic effect. In the FPY 14 strain, the activities of esterase, GST and P450 enzymes were 1.92-, 1.06- and 3.96-fold higher, respectively, when compared to the susceptible GSS strain. The P450 and esterase enzymes might play a role in the mechanism of resistance to fenpyroximate.     

The inheritance of resistance to soft rot (Erwinia carotovora subsp.atroseptica) in diploid potato families

Soft rot of potato tubers, caused byErwinia (Pectobacterium), is a serious disease affecting potato crops during storage. Studies on the inheritance of resistance to tuber soft rot were undertaken on diploid potato hybrids. A total of 480 clones, derived from 12 families, were examined for resistance to soft rot in laboratory tests over 3 years and for basic agronomic traits in field experiments over 2 years. General (GCA) and specific (SCA) combining abilities were significant in the inheritance of resistance to soft rot; however, GCA for female parents were significant in 1 or 2 years during 3 years of evaluation and SCA was significant for eight, two, and six families out of 10 tested in 1994, 1995, and 1996, respectively. There were also significant differences between years. Moreover, marked interactions for GCA_females × year and SCA × year in the variation of soft rot resistance were found. Broad-sense and narrowsense heritability of resistance, measured as diameter of rotten tissue, was estimated as 0.92 and 0.89, respectively. The maternal effect, evaluated in two sets of reciprocal crosses, was not significant for the genetic determination of resistance to soft rot. No significant relationships were found between resistance to soft rot and the main agronomic traits in the tested diploid families. These results suggest that diploid potato clones with resistance to soft rot can be selected after being evaluated over a few seasons. It is also possible to select clones combining good resistance to soft rot with high yield, superior tuber characteristics, and acceptable starch content. These resistant diploids can be used as male parents in 4x-2x crosses in breeding tetraploid potato resistant to soft rot.     

玉米禾谷镰孢穗腐病抗性的QTL定位

以3个高抗玉米自交系承351、丹598、吉V203和1个高感自交系ZW18为亲本,分别构建3个F₂群体及其对应的F_2:3家系,通过对F_2:3家系发病果穗进行图像处理的方法鉴定抗病表型,对禾谷镰孢穗腐病抗性进行QTL定位。3个F₂群体共检测到11个与禾谷镰孢穗腐病抗性有关的QTLs,分别可解释4.87%～40.98%的表型变异率。来源于抗病亲本承351的QTL-qRgr7-1位于7.02 bin,可解释高达13.76%～40.98%的表型变异率。通过与前期病害评级方法定位到的QTLs相比,qRgr7-1在Bins7.02上和qRger7.1完全重合,qRgr2-1在Bins2.01-2.02上和qRger2.1完全重合,qRgr10-1在Bins10.01-10.02上和qRger10.1完全重合。基于图像分析定位到的qRgr9-1、qRgr1-2、qRgr3-1分别与基于抗病评级定位到的区间存在重叠区域。利用不同方法定位到了相同的QTLs,一方面说明基于图像分析进行表型鉴定有一定的准确性,另一方面也验证了这些QTLs位点的真实性。     

Analysis of QTL for resistance to head smut (Sporisorium reiliana) in maize

Head smut of maize, caused by the fungus Sporisorium reiliana, is an important disease in the temperate maize-growing areas worldwide. In this study, we mapped and characterized quantitative trait loci (QTL) conferring resistance to S. reiliana using a F_2:3 population of 184 families derived from a cross between Mo17 (resistant parent) and Huangzao4 (susceptible). The population was evaluated for resistance in replicated field trials with artificial inoculation of S. reiliana chlamydospores in Gongzhuling of Jilin Province and Harbin of Heilongjiang Province of China, two hot spots of head smut incidence, in 2003 and 2004. Genotypic and G × E variances for disease incidence were highly significant in the population. Heritability estimates for percentage disease incidence in the 2-location and 2-year evaluation ranged from 0.62 to 0.70. Composite interval mapping on a linkage map (1956.1 cM distance; 9.34 cM average interval) constructed with 84 SSR and 135 AFLP markers, identified five QTL, one each on chromosomes 1, 3 and 8 and two on chromosome 2, accounting for 5.0–43.7% of the phenotypic variance across four environments. One major QTL on chromosome 2 explaining up to 43.7% of the phenotypic variance can potentially be used in molecular marker-assisted selection for head smut resistance in maize.     

Rapid in-season detection of herbicide resistant Alopecurus myosuroides using a mobile fluorescence imaging sensor

WeedPAM has been introduced as a new chlorophyll fluorescence imaging sensor to detect herbicide stress in weeds a few days after treatment (DAT). In this study, it was investigated if the sensor could differentiate between 50 sensitive and herbicide resistant populations of Alopecurus myosuroides 5 DAT. Resistance profile of all populations had been analyzed in standard greenhouse bioassays. Populations were sown in winter wheat at several locations in Germany over two years. At 3–7 leaves growth stage, they were treated with four ALS- and three ACCase-inhibitors at recommended dosages. Five DAT, maximum quantum efficiency of PS II was measured with the WeedPAM sensor on 40 A. myosuroides plants per treatment. Based on the sensor data, populations were classified into sensitive and resistant populations. Classification was verified by a visual assessment of all treatments and populations 21 DAT. In total, 95% of the WeedPAM classifications 5 DAT were correct. We could demonstrate that WeedPAM is capable to detect herbicide resistant A. myosuroides populations shortly after treatment. This allows selecting alternative weed control methods against resistant weed populations in the same growing season.     

Inheritance of the resistance toGlobodera rostochiensis pathotype Ro2 in potato

Nine advanced tetraploid potato clones withS. tuberosum ssp.tuberosum, S. tuberosum ssp.andigena, andS. vernei background were found to be highly resistant toGlobodera rostochiensis pathotype Ro2. In order to characterize the inheritance of the resistance to Ro2 and determine the genotype of these Ro2-resistant clones, a series of testcrosses to susceptibleS. tuberosum cultivars, as well as crosses between the resistant clones, were made. A total of 1081 seedlings corresponding to 18 families were grown, inoculated with 3000G. rostochiensis juveniles (J2), and evaluated by the root-ball method. To determine which genetic model best fit the data, goodness-of-fit-tests were performed. The analysis assumed tetrasomic inheritance under the hypothesis that resistance is due to one locus with complete dominance. From the analysis of the data it is concluded that there is no consistent evidence against the hypothesis that a single dominant gene is associated with the resistance to Ro2 and that the Ro2-resistant clones are simplex. In general, the number of cysts observed on the root balls was low compared with the results of other studies, the results of the different inoculations were variable, and there was an excess of resistant plants. Possible explanations for these observations are discussed.     

Phenotypic and Genomic Modifications Associated with <Emphasis Type="Italic">Globodera pallida</Emphasis> Adaptation to Potato Resistances

Studying phenotypic and genomic modifications associated with pathogen adaptation to resistance is a crucial step to better understand and anticipate resistance breakdown. This short review summarizes recent results obtained using experimentally evolved populations of the potato cyst nematode Globodera pallida. In a first step, the variability of resistance durability was explored in four different potato genotypes carrying the resistance quantitative trait loci (QTL) GpaV_vrn originating from Solanum vernei but differing by their genetic background. The consequences of the adaptation to resistance in terms of local adaptation, cross-virulence and virulence cost were then investigated. Finally, a genome scan approach was performed in order to identify the genomic regions involved in this adaptation. Results showed that nematode populations were able to adapt to the QTL GpaV_vrn, and that the plant genetic background has a strong impact on resistance durability. A trade-off between the adaptations to different resistant potato genotypes was detected, and we also showed that adaptation to the resistance QTL GpaV_vrn from S. vernei did not allow adaptation to the colinear locus from S. sparsipilum (GpaV_spl). Unexpectedly, the adaptation to resistance led to an increase of virulent individual’s fitness on a susceptible host. Moreover, the genome scan approach allowed the highlighting of candidate genomic regions involved in adaptation to host plant resistance. This review shows that experimental evolution is an interesting tool to anticipate the adaptation of pathogen populations and could be very useful for identifying durable strategies for resistance deployment.     

Foliar resistance to ozone injury in the genetic base of U.S. and Canadian soybean and prediction of resistance in descendent cultivars using coefficient of parentage

Development of ozone (O₃)-resistant cultivars is a potentially important approach for maintaining crop productivity under future climate scenarios in which tropospheric O₃ pollution is projected to rise. A first step in the breeding of resistant cultivars for a crop such as soybean (Glycine max (L.) Merr.) is identification of sources of O₃ resistance genes. Thirty ancestral lines of soybean were screened for differences in O₃ foliar injury under greenhouse conditions. The ancestors represented 92% of the genetic base of North American soybean as determined by pedigree analysis. Injury among ancestors ranged from 5 to 50% of leaf area, based on response of the five oldest main stem leaves, indicating both the presence of substantial genetic variation for O₃ injury among the ancestors as well as resistance levels greater than that of the standard control cultivar, resistant Essex (15% injury). Ancestral types Fiskeby 840-7-3 and Fiskeby III exhibited the greatest foliar resistance and PI 88788 the least. A subsequent field study confirmed the foliar resistance of the Fiskeby types. Resistant ancestors identified here are proposed for inheritance and DNA mapping studies to determine the genetic basis of foliar resistance. Because the presence of O₃-resistant ancestors suggested that resistant descendents may exist in addition to the resistant control Essex, a method was developed to facilitate their identification. A predicted O₃-resistance score was calculated for 247 publicly-released cultivars, based on pedigree analysis and ancestral response to ozone. Using this approach, the 32 public cultivars most closely related to resistant ancestors and, thus, most likely to be resistant were identified as priority candidates for future screening efforts. Predicted scores from the analysis suggested that cultivars from the Midwest may be more sensitive to foliar injury, on average, than Southern cultivars.     

利用农杆菌介导合子胚转化法将Bar基因转入玉米自交系的研究

基于农杆菌介导的合子胚转化方法是一种具有自主知识产权的转基因新方法。利用该方法转化玉米合子胚,可直接获得转化种子。以东北春玉米区玉米自交系8902、Pa91、丹340作为转化受体,通过合子转化法进行抗草丁膦除草剂基因Bar的转基因研究。在获得的3 560份材料中,通过对T_1代种子苗叶面喷施草丁膦除草剂Basta,共获得153株抗性植株,抗性频率为4.3%。PCR鉴定阳性植株为102株,转化频率为2.87%。对PCR检测出的阳性植株进行自交获得T_2代,对材料继续进行抗性筛选,对抗除草剂表型的分离比率进行抗性遗传分析。     

Inheritance of the resistance toMeloidogyne incognita,M. javanica,andM. arenaria in potatoes

The objectives of this research were: 1) to try to determine the inheritance of the resistance toMeloidogyne incognita, M. javanica, andM. arenaria and 2) to study genetic relationships among the three species of root-knot nematodes in potatoes. The source of resistance used wasSolanum sparsipilum. The progenies analyzed in this study may be grouped into a few discrete segregation patterns, three forM. incognita andM. javanica and two forM. arenaria. More than one dominant locus, but not many loci are needed to explain the segregation obtained in the three species. Susceptible clones ofS. sparsipilum possess recessive or epistatic genes which affect the pattern of segregation for resistance, when these clones are crossed to resistant clones. Maternal effects were expressed in reciprocal crosses between susceptible and resistant clones when they were tested for resistance toM. arenaria. Clones in this study which are resistant toM. arenaria are also resistant toM. incognita andM. javanica     

Diclofop resistance in sterile wild oat (Avena sterilis L.) in wheat fields in Greece and its management by other post-emergence herbicides

In 2009, a survey was conducted of cereal fields in five prefectures of Greece to establish the frequency and distribution of herbicide-resistant sterile wild oat (Avena sterilis L.). In total, 104 sterile wild oat accessions were collected and screened in a field experiment (conducted in 2009 and repeated in 2010) with several herbicides commonly used to control this weed. Most of the sterile wild oat accessions (89%) were classed as resistant (or developing resistant) to the ACCase-inhibiting herbicide diclofop, while resistance to other ACCase-inhibiting herbicides was markedly lower. The results of the pot experiments showed that some of the sterile wild oat accessions were found to have a very high level of diclofop resistance (resistance index up to 28.6), while cross-resistance with other herbicides was common. The levels of resistance and cross-resistance patterns varied among biotypes with different amount and time of selection pressure, indicating either more than one mechanism of resistance or different resistance mutations in these sterile wild oat biotypes. LA14, which had the highest diclofop resistance level (28.6 resistance index), showed resistance to all APP herbicides applied and non-ACCase inhibitors. Alternative ACCase-inhibiting herbicides, namely tralkoxydim and pinoxaden remain effective on 86 and 92% of the tested sterile wild oat populations, respectively. For the ALS-inhibiting herbicide mesosulfuron + iodosulfuron, nearly all the sterile wild oat accessions were susceptible (97%), with only 3 of them classed as developing resistance. Therefore, there is an opportunity to effectively control sterile wild oat by selecting from a wide range of herbicides and other cultural practices. Early post-emergence herbicide application is strongly suggested, since it could decrease the number of resistant seeds in the field and slow down the dispersal of this major problem.     

Inheritance of resistance to sclerotinia stem rot (Sclerotinia trifoliorum) in faba beans (Vicia faba L.)

Stem rot, a fungal disease caused by Sclerotinia trifoliorum Eriks., is often a serious problem in many important forage legumes including faba beans (Vicia faba L.). Understanding the inheritance of resistance to the disease is essential for effective breeding of resistant cultivars. Experiments were conducted to study the inheritance of resistance to stem rot of faba beans. The F₁, F₂, and the backcross generations of five crosses between four resistant and four susceptible populations (Alto × Polycarpe, A-90 × Polycarpe, ILB-1814 × A-247, A-90 × A-244, VT × Tanagra) were used. The eight populations were crossed properly in the field, and progenies of F₁ and F₂, as well as backcross progenies of F₁ with each of their parents, were evaluated for resistance to stem rot disease under controlled conditions after artificial inoculation of the plants with carrot root pieces colonized by the fungus. On the assumption that inheritance of stem rot resistance is governed by a single dominant gene, no significant differences were found between the observed and the expected frequencies of resistance for progenies, except for one cross. As the expression of resistance to the disease fits the expected ratios for a single dominant gene model, it is concluded that the inheritance of resistance to sclerotinia stem rot in the evaluated faba bean populations is controlled by a single dominant gene.     

Genetic analysis,realized heritability and synergistic suppression of indoxacarb resistance in Phenacoccus solenopsis Tinsley (Homoptera: Pseudococcidae)

Cotton mealybug Phenacoccus solenopsis Tinsley (Homoptera: Pseudococcidae) has caused huge crop and economic losses in different countries of the world including Pakistan. Considering P. solenopsis as a key cotton pest, research was conducted to understand genetics, realized heritability and preliminary mechanism of indoxacarb resistance. As a result of selection with indoxacarb, P. solenopsis developed a 2223.13-fold resistance after five generations. Indoxacarb resistance was autosomal in nature, and the degree of dominance for F₁ and F₁′ was 0.60 and 0.62, respectively, indicating the incomplete dominance of indoxacarb resistance. A monogenic model of inheritance also showed the involvement of multiple loci in indoxacarb resistance in P. solenopsis. Realized heritability for indoxacarb resistance was markedly high (h² = 1.13) in P. solenopsis suggesting how quickly resistance can develop as a result of selection. Synergism tests with piperonylbutoxide (PBO) and S,S,S-tributylphosphorotrithioate (DEF) also suggested that indoxacarb resistance in P. solenopsis was metabolism-based due to possible involvement of both mono-oxygenases and esterases. Hence, it was concluded that indoxacarb resistance in P. solenopsis was autosomal, incompletely dominant, polygenic and metabolism-based. These findings will be helpful in the management of P. solenopsis.